Hugs and other physical contact between households will be permitted for the first time since restrictions began more than a year ago.
But the measures are being eased as top scientists warn of a “perilous moment”, with the Indian variant feared to be as much as 50% more transmissible than the Kent strain.
In a statement, Mr Johnson said: “Together we have reached another milestone in our roadmap out of lockdown, but we must take this next step with a heavy dose of caution.
“We are keeping the spread of the variant first identified in India under close observation and taking swift action where infection rates are rising.”
He said the “current data does not indicate unsustainable pressure on the NHS”, and that second vaccine doses are being accelerated to give the greatest protection to the most vulnerable.
“But now everyone must play their part – by getting tested twice a week, coming forward for your vaccine when called and remembering hands, face, space and fresh air,” he added.
“I urge everyone to be cautious and take responsibility when enjoying new freedoms today in order to keep the virus at bay.”
Ministers are hoping surge testing and vaccines will allow a safe opening up of the nation, with jabs due to be extended to the over-35s this week.
He said there are more than 1,300 cases of the Indian variant of concern, which is “relatively widespread in small numbers” and is becoming “the dominant strain” in Bolton and Blackburn.
But offering good news over plans to ease restrictions without unleashing a fresh wave of infections and deaths, Mr Hancock said there is “new very early data” from Oxford University giving confidence that existing vaccines work against the variant.
“That means that we can stay on course with our strategy of using the vaccine to deal with the pandemic and opening up carefully and cautiously, but we do need to be really very vigilant to the spread of the disease,” he told Sky’s Sophy Ridge On Sunday.
“We have a high degree of confidence that the vaccine will overcome.”
Cinemas, hotels and B&Bs will also be allowed to reopen on Monday, as the “rule of six” will be applied to indoor meetings, while physical distancing ends between friends and family.
The easing comes after official figures showed more than 20 million people have received both vaccine doses, covering more than 38% of UK adults, while more than 69% had received at least one.
Sir John Bell, Oxford’s regius professor of medicine, said the result of lab experiments investigating whether the vaccine neutralises the Indian variant “looks okay”.
“It’s not perfect, but it’s not catastrophically bad,” he told Times Radio, adding there is only “a slight reduction in the ability to neutralise the virus”.
Mr Hancock urged people to come forward for their jabs when eligible as he said the “vast majority” of people hospitalised with the strain in Bolton have not taken the vaccine.
The Health Secretary said there had been no known deaths from the Indian variant in Bolton of somebody who has received both jabs.
Five people have been in hospital with it after receiving their first vaccine dose, while one person who had received both doses had been admitted “but that person was frail”, he told The Andrew Marr Show.